Adding my data point to the list here, my circumstances are almost idential to the OP's.
Win Server Std 2012R2 backing up to a usb hard disk then duplicated out to an LTO6 Tape drive. Duplicates and normal backups to this tape drive are failing ...
> They said they can't help me because this version of Backup Exec has reached end-of-life
I see you have managed to find a solution which is great. I just had to leave a comment though. I would be prepared to kick up an absolutely enormous stink ...